1. Technical Field of the Invention
The present invention relates to ceramic bodies and ceramic catalyst bodies and, more particularly, to a ceramic body, a ceramic catalyst body and related manufacturing methods, with the ceramic body having suitable applications to a three-way catalyst for motor vehicle, a combustion catalyst for gas turbine and a high-temperature purifying catalyst which are exposed to high temperatures above 800° C. and high-velocity gas streams for removing harmful compounds such as NOx or the like.
2. Description of the Related Art
In modern automotive engines, catalyst bodies such as three-way catalysts have heretofore been widely used for removing toxic compounds from exhaust gases emitted from internal combustion engines. Each three-way catalyst comprises an oxide-based honeycomb structural body for supporting a three-way catalyst. An attempt has heretofore been made to provide a cordierite porous body for use in the honeycomb structural body. The cordierite porous body is exposed to high temperatures of exhaust gases for prolonged periods of time with the resultant deterioration in a specific surface area due to a sintering effect.
In the related art, efforts have been devoted to developing the oxide-based honeycomb structural bodies for three-way catalysts, which have been put into practical application to the three-way catalyst for motor vehicles and combustion catalysts, etc., which are exposed to high temperatures for prolonged periods of time. Research and development work has been undertaken to provide oxide-based honeycomb structural bodies with further improved characteristics. Among these, especially, a cordierite is known as having a melting point of about 1400° C. with an extremely low thermal expansion coefficient and increased thermal shock resistance. Owing to these characteristics, the cordierite has been employed in honeycomb structural bodies as, for instance, catalyst carriers, exposed to high temperature areas at temperatures exceeding 800° C., for a three-way catalyst of a motor vehicle, a combustion catalyst for a gas turbine or a catalyst for purifying high temperature gases.
Thus, even though application availability of the cordierite as the catalyst carrier has been recognized in the related art, the related art manufacturing method is hard to manufacture a cordierite porous body having a high specific surface area and thermally stable characteristic. To overcome such a difficulty, efforts have been undertaken to provide a cordierite honeycomb structural body with increased thermal shock resistance as an exhaust gas purifying catalyst. The cordierite honeycomb structural body, having a catalyst carrier surface coated with a layer of gamma alumina and supporting a noble metal catalyst, has been widely used. The reason for the coated layer to be formed on the catalyst carrier surface resides in that the cordierite has a small specific surface area and has a difficulty of supporting a catalyst component in required amounts with the surface structure remaining intact and the use of material with an increased specific surface area such as gamma alumina enables an increase in the specific surface area of the catalyst carrier.
However, coating a cell wall surface of the catalyst carrier with gamma alumina results in an increase in weight accompanied with an increase in thermal capacity. In recent years, various studies have heretofore been undertaken to decrease a thickness of the cell wall to lower the heat capacity with a view of activating the catalyst on an earlier stage. With the coated layer being formed, an issue occurs with an effect of the cell wall decreasing by half. Thus, a need has been arisen to improve such a defect. Further, with the coated layer being formed on the cell wall surface, a remarkable reduction occurs in an opening space of each cell, causing an increase in a pressure loss. In addition, each cell has a greater thermal expansion coefficient than that of the catalyst carrier merely composed of the cordierite. Moreover, a transition takes place from gamma alumina to alpha alumina when the cell is exposed to high temperatures above 1000° C. In addition, with the cell exposed to such high temperatures, a sintering effect proceeds on the cell, causing an issue to occur with a difficulty of keeping a high specific surface area.
Research and development work has been done with the present inventors in success to provide cordierite porous bodies each formed with cordierite acicular crystals in the order of submicron diameters as disclosed in Japanese Patent Application Publication No. 2003-321280, U.S. Patent Application Publication Nos. 2003/151155, 2004/131512, U.S. Pat. No. 6,887,826, Japanese Patent Application Publication No. 2002-119870, U.S. Pat. No. 7,067,452 and EP1043067A2. These include the honeycomb structural bodies each including the cordierite porous body, which is directly utilized intact, and the honeycomb structural bodies each including the cordierite porous body whose inner wall has a coated layer.
There has been no choice but to employ a method of coating a layer of gamma alumina or the like on the inner wall of the honeycomb structural body when using the cordierite to an area exposed at high temperatures. Therefore, various efforts have heretofore been devoted to developing a ceramic body that can support a catalyst component with no formation of the coated layer. One of these technologies is disclosed in Japanese Patent Application Publication No. 62-004441. With a manufacturing method disclosed in this technology, a cordierite body is prepared and subjected to, for instance, acid treatment, upon which the cordierite body is subjected to heat treatment, thereby causing the cordierite body to have an increased specific surface area. However, such a manufacturing method undergoes an issue with the occurrence of deteriorations in structure wherein a crystal lattice of the cordierite are ruptured due to acid treatment and heat treatment with the resultant reduction in strength. Thus, such a cordierite body has been impractical as a catalyst carrier.
To address such an issue, the present inventors have proposed a ceramic carrier with a structure as disclosed in U.S. Pat. No. 7,048,894. With the ceramic carrier disclosed in this Patent Publication, the structure of the ceramic carrier has a capability of supporting a catalyst component in required amounts with no need for a coated layer to be formed for providing an improved specific surface area.
The ceramic carrier is composed of substrate ceramic materials including constituent elements and at least one kind of or multiple kinds of the constituent elements a part of which are replaced by other elements than the constituent elements of the substrate ceramic material. That is, the ceramic carrier is dipped into a solution of noble metal compounds such as hexachloroplatinic acid, platonic chloride, rhodium chloride, etc. Then, the ceramic carrier is fired, thereby making it possible to directly support the noble metal catalyst on the replaced elements. This ceramic carrier has higher strength and durability than those of the related art catalyst carrier formed with vacant pores upon acid treatment and heat treatment.
Another attempt has been made to provide a ceramic catalyst as proposed in U.S. Patent Application Publication No. 2003/109383. With the ceramic catalyst disclosed in this Patent Publication, the ceramic catalyst includes a ceramic carrier capable of directly supporting a catalyst component. The ceramic carrier has a carrier surface on which a main catalyst component and sub catalyst components are directly supported. In directly supporting the main catalyst component and the sub catalyst components on the carrier surface, the main catalyst component is supported on the carrier surface in advance on which the sub catalyst components are subsequently supported, thereby providing a catalyst body that is hard to suffer thermal deactivation.
Under such circumstances, the present inventor have dedicated their keen efforts on accumulative research and development work toward a goal of developing a catalyst-support cordierite honeycomb structure that makes it possible to achieve a radical solution on various issues encountered in the related art on the ground of the related art technologies.
As a result of such accumulative research and development work, the present inventors have developed new insights on improvement of the catalyst-support cordierite honeycomb structure.
That is, acicular particles can be developed on a cordierite base material on a particular technology. Then, coating a part of or a whole of the acicular particles with a constituent element different from that of the acicular particles makes dramatic improvements in a specific surface area.
Such a structure allows the acicular particles to be stable even under high temperature environments. This makes it possible to dramatically suppress the occurrence of a reduction in the specific surface area due to a sintering effect. Coating surfaces of the acicular particles with a layer capable of supporting a catalyst enables the realization of a ceramic catalyst body having an excellent catalytic performance. This makes it unnecessary to apply the inner wall of the honeycomb structure with a layer of gamma alumina as required in the related art manufacturing method.